The University of Minnesota team produced a microchip containing a large array of graphene electronic tweezers. Fluorescence images show DNA molecules and polystyrene nanoparticles trapped on the chip. (Image courtesy of Barik et al./University of Minnesota.)

World's Sharpest Tweezers

The physical principle of tweezing or trapping nanometer-scale objects, known as dielectrophoresis, has been known for a long time and is typically practiced by using a pair of metal electrodes. From the viewpoint of grabbing molecules, however, metal electrodes are very blunt. They simply lack the "sharpness" to pick up and control nanometer-scale objects.

"Graphene is the thinnest material ever discovered, and it is this property that allows us to make these tweezers so efficient. No other material can come close," said research team leader Sang-Hyun Oh, a professor in the University of Minnesota's department of electrical and computer engineering. "To build efficient electronic tweezers to grab biomolecules, basically we need to create miniaturized lightning rods and concentrate huge amount of electrical flux on the sharp tip. The edges of graphene are the sharpest lightning rods."

The team also showed that the graphene tweezers could be used for a wide range of physical and biological applications by trapping semiconductor nanocrystals, nanodiamond particles and even DNA molecules.

Normally, this type of trapping would require high voltages, restricting it to a laboratory environment, but graphene tweezers can trap small DNA molecules at around 1 Volt, meaning that this could work on portable devices such as mobile phones. This could be key to meeting the grand challenge of engineering better personalized medicines.

Using the nanofabrication facilities at the Minnesota Nano Center, electrical and computer engineering professor Steven Koester's team made the graphene tweezers by creating a sandwich structure where a thin insulating material—hafnium dioxide—is sandwiched between a metal electrode on one side and graphene on the other.

Atomically sharp edges of electrically driven graphene can act as 'tweezers' that rapidly trap biomolecules from the surrounding solution. (Image courtesy of In-Ho Lee/University of Minnesota.)

"One of the great things about graphene is it is compatible with standard processing tools in the semiconductor industry, which will make it much easier to commercialize these devices in the future," said Koester, who led the effort to fabricate the graphene devices.

"Since we are the first to demonstrate such low-power trapping of biomolecules using graphene tweezers, more work still needs to be done to determine the theoretical limits for a fully optimized device," said Avijit Barik, lead author of the study. "For this initial demonstration, we have used sophisticated laboratory tools such as a fluorescence microscope and electronic instruments. Our ultimate goal is to miniaturize the entire apparatus into a single microchip that is operated by a mobile phone."

Graphene Tweezers

Another feature that separates graphene tweezers from metal-based devices is that graphene can also "feel" the trapped biomolecules. In other words, the tweezers can be used as biosensors via simple electronic techniques.

"Graphene is an extremely versatile material," Koester said. "It makes great transistors and photodetectors, and has the potential for light emission and other novel biosensor devices. By adding the capability to rapidly grab and sense molecules on graphene, we can design an ideal low-power electronics platform for a new type of handheld biosensor."

Oh agrees that the possibilities are endless.

"Besides graphene, we can utilize a large variety of other two-dimensional materials to build atomically sharp tweezers combined with unusual optical or electronic properties," said Oh. "It is really exciting to think of atomically sharp tweezers that can be used to trap, sense, and release biomolecules electronically. This could have huge potential for point-of-care diagnostics, which is our ultimate goal for this powerful device."